1. Field of the Invention
The present invention relates to a damping force generation device for a vehicle, and more particularly, to a damping force generation device including a shock absorber.
2. Description of the Related Art
When a vehicle such as an automobile travels, static electricity is generated in the vehicle due to the flow of air in a state in which the air is brought into friction contact with the vehicle. Static electricity is also generated when each portion of a tire is repeatedly brought into contact with a road surface and separated therefrom along with the rotation of a wheel, when components in an engine, a brake device, or the like move relatively under a contact state, and the like.
The vehicle is substantially electrically insulated from the ground due to a tire having low conductivity, and hence electric charge (in general, positive electric charge) is charged to a vehicle body or the like when static electricity is generated in the vehicle. Radio noise is liable to be generated when the electric charge is charged to the vehicle body or the like. Therefore, a structure for reducing electric charge that is charged to a vehicle through the passage of an electric current has been investigated hitherto, and various structures have been proposed.
For example, in Japanese Patent Application Laid-open No. 2009-181694, there is disclosed an electrostatic eliminator having such a configuration that charged silicon is filled into a case in which ceramic bodies are arranged densely in a radial fashion, and one of conductive wires connected to both sides of the case is connected to a minus terminal of a battery and the other conductive wire is connected to a vehicle body. In this type of the electrostatic eliminator, static electricity of the vehicle body is neutralized by grounding, and thus the electric charge that is charged to the vehicle body can be reduced.
The related-art electrostatic eliminator as disclosed in Japanese Patent Application Laid-open No. 2009-181694 has a complicated structure, and hence the electrostatic eliminator is required to be connected to the minus terminal of the battery and the vehicle body through the conductive wires, with the result that an installation space for the electrostatic eliminator is also required.
Incidentally, the vehicle includes damping force generation devices configured to damp vibrations of the vehicle body. Each of the damping force generation devices includes a shock absorber having a cylinder and a piston fitted to the cylinder in a reciprocally movable manner. The shock absorber is coupled to a sprung member and an unsprung member of the vehicle at a rod part of the piston and the cylinder, respectively, and generates the damping force due to a flow resistance when oil passes through orifices formed in the piston along with relative displacement between the sprung member and the unsprung member.
Experimental research conducted by the inventors of the present invention has revealed that an adverse effect on the vehicle exerted by electric charge that is charged to the vehicle is not limited to the increase in the risk of generation of radio noise. In other words, when the electric charge is charged to the vehicle, the electric charge is also charged to the oil in the shock absorbers. As a result, the viscosity of the oil is increased so that damping forces are liable to become excessive.
The related-art electrostatic eliminator as disclosed in Japanese Patent Application Laid-open No. 2009-181694 is restricted in terms of an installation location, and cannot thus be installed at a location for effectively reducing the electric charge that is charged to the oil in the shock absorbers. Therefore, the excessive damping force resulting from the increase in the viscosity of the oil caused by the charging of the electric charge cannot be effectively prevented by the related-art electrostatic eliminator.